Started my first run today

I started my first run today as i always felt the original games were lacking a lot. I wanted to experience the same feeling again i got with playing gen 1-4. I already ran into so much different mons in my first hours which is absolutely nuts and i like the game being harder (i play on difficult)

I have a few questions and cant find proper answers.

-Can i visit other regions later?

-I know the region has 8 gyms and elite four but what are the main features of the late game?

-I read about a dex size of 906 somewhere, is this really true?

Thanks for your help and i really would appreciate also other forms of tips and tricks you have for me :)